The inner surface of a pneumatic rubber tire is typically composed of a rubbery, elastomeric composition designed to prevent or retard the permeation of air and moisture into the carcass from the tire's air chamber. The portion of the tire containing such inner surface is often referred to as an inner liner. Inner liners have been used for many years in tubeless, pneumatic vehicle tires to retard or minimize the escape of air used to inflate the tire, thereby maintaining tire pressure.
Rubbers which are relatively impermeable to air are often used as a major portion of such inner liners. These can include butyl and halobutyl (e.g. chlorobutyl and bromobutyl) rubbers. U.S. Pat. No. 3,808,177 discloses other polymers which may also be relatively impermeable.
The inner liner forms the inner surface of a tire and has, as an important purpose, to reduce the air permeability of the tire. Typically, the inner liner is applied to the inner surface of the tire carcass while both are in the treen or uncured state. When the tire is cured, such inner liner becomes an integral, co-cured part of the tire. Other techniques are used in some cases; for example, U.S. Pat. No. 3,299,934 discloses application of a cured polyurethane in the form of a solution to the inner surface of a tire.
Rubber composition-based inner liners add both weight and cost to the manufacture of a tire. Relatively thick inner liner coatings are generally necessary to obtain the desired degree of air impermeability; the added weight and cost are largely attributable to the thickness of coating desired.
Rubber tires having a thin coating of polyvinylidene chloride on the inner surface of the tire for the purpose of reducing the tire's air permeability are known. Such coatings are disclosed, for example, in United Kingdom published patent application GB 2 023 516A and in Japan patent publication No. 31761/1972. The latter reference also discloses vinylidene chloride copolymer coatings for the same purpose.
Polyvinylidene chloride (PVDC) and copolymers of vinylidene chloride (VDC) with other monomers (e.g., vinyl chloride and acrylonitrile) are thermoplastic materials which are known for their gas and vapor barrier properties. For example, VDC copolymer film is sold commercially under the name, "Saran," a trademark of Dow Chemical Co., for example for use as a wrapping material where low oxygen and water vapor permeability are required (in food wrapping, for example). However, PVDC and VDC copolymers tend to be brittle. Thus, PVDC coatings on the inner surfaces of tires, such as those described in the aforesaid United Kingdom and Japan patent applications, are understood to be brittle. They have low elongation (a characteristic of brittle materials) and low flex resistance. Thus, although they impart low air permeability to the tire while the tire is new, they do not stand up under the normal flexing which a tire must undergo in service, and they crack and flake off long before the tire carcass and tread reach the end of their useful lives.
Blends of butyl rubber (maximum 10 percent by weight) with "Saran" (a vinylidene chloride-vinyl chloride copolymer containing a maximum of 5 weight percent of additional comonomers) and an epoxy resin, and packaging films made therefrom, which films are characterized by low oxygen permeability, and disclosed in U.S. Pat. No. 4,239,799. "Modern Plastic Encyclopedia 1984-1985," pages 94 and 96, disclosed that "Saran" may be alloyed with elastomeric materials to provide toughness down to minus 30.degree. C. Neither proposes use of a "Saran"/rubber blend in tires. "Modern Plastics Encyclopedia" further discloses that VDC homopolymer has a melting point of 198.degree.-205.degree. C. and begins to decompose rapidly at 210.degree. C., making it difficult to process, and that comonomers typically depress the melting point, making melt processing feasible.